1. Field of the Invention
This invention pertains to the cantilevered journal friction bearings used to rotatively support the rock cutting cones of rotary rock bits. More specifically, this invention is directed to the methods of construction and assembly of such journal bearings.
2. Brief Summary of the Prior Art
Early examples or rotary rock bits used bearings formed of the parent metal of the journal shaft and of the rotating cone. Depending upon the skill of the manufacturer, some such bearings had flame hardened surfaces.
The continuing search for oil requires drilling ever deeper wells, demanding more of the rock bit and its' bearings. Hard facing metals, as first applied to the rock bit teeth, were later applied to the bearing surfaces of the journal shafts.
Many friction bearing designs are regarded as being used to bear loads predominantly or exclusively in one direction. The "unloaded" side is freely used to reservoir lubricant to produce the hydro-dynamic lubricant film which serves to separate the bearing surfaces on the "loaded side".
One such bearing is described in U.S. Pat. No. 3,995,917 by Quinlan. In production, most of the length of the radial bearing portion of the journal shaft is undercut in relief. Subsequently, the "loaded" side is filled with arc deposited hard facing metal, then ground to finished diameter. The unloaded side is left unfilled, providing a ready storage location for lubricant. The cooperating radial bearing portion of the bore of the rotary cutter is undercut in a similar manner and filled with arc deposited aluminum bronze and finish ground to size.
In a competing design, the unloaded side of the hardened journal shaft bearing area is relieved on the unloaded side for the storage of lubricant by means of an eccentric grinding operation. No hard facing material is added to this bearing.
Yet another popular design features a hard metal filled relief on the loaded side of the journal shaft, the shaft being then ground full round. The radial bearing portion of the cone bore is relieved over about half of its area, leaving axially oriented narrow lands of steel spaced apart by similarly shaped reliefs. The reliefs are filled with arc deposited soft bearing metal, and the bore is then ground to size. While the hard steel lands resist wear and support the load, the soft bars aid in load support and also serve to trap detritus by embedding.
Another bearing surface of similar nature but with a unique method of production has been patented in this country for use in a rock bit. The steel bearing surface is formed to near the finished diameter, deeply knurled, and subsequently over coated with a fused deposit of softer bearing alloy. The bearing is then ground to size, producing a surface of small work hardened diamond shaped areas of steel within a grid of soft bearing alloy.
Mayo, U.S. Pat. No. 3,721,307, describes a rock bit bearing using a floating bushing of beryllium copper running between hardened steel bearing surfaces of the journal and the cone bore. The bushing is about 0.125 thick thus occupying much valuable space. Because this design behaves like a bearing within a bearing, the running clearances are effectually doubled. Both the inner and the outer bearing run beryllium copper against hardened steel, a bearing couple of known advantage.
Murdoch, U.S. Pat. No. 3,917,361, teaches the use of a thin, split, floating, flexible bushing in a rock bit journal. Beryllium copper is specified as preferred, providing the same couple as Mayo. The split ring compensates for thermal changes in its own diameter providing for closer running clearances.
Murdoch, U.S. Pat. No. 3,990,751, uses a floating bushing of maraging steel, for high strength and toughness, plated with 0.001 . inch of silver for a desired bearing couple in unison with the hard steel of the cone and the journal.
A commercial variation on the floating bushing uses a similar bushing being force fit into the cone bore. The running clearance is normal. The bushing is thick to withstand the assembly and running loads, and the danger of metal chips caused by shearing during the pressing operation is noted. One such bearing is described by Mayo et al, U.S. Pat. No. 4,293,167.
Rock bits typically are used until some part fails. Ideally, the bearings should hold up until the cutting teeth wear out, and the bit may then be withdrawn without leaving metal parts in the well bore. Bearings, and/or their grease seals, generally fail first, sometimes resulting in the down hole loss of rotary cones. A need exists, therefore, for further improvement in the bearing systems of rock bits aimed at lengthening the reliable life expectancy of such bearings in service. Many bearing couples are superior to the heat-treated steels of cone and journal running against each other, however, providing alternate metals generally involves compromising some other desired features; structure, strength/space, or running clearance.
The production of fine finishes and close running fits are important considerations in service longevity, not well supported by such practices as relieving the unloaded side of the journal shaft bearing, or by the use of a floating bushing which also serves to double the running clearances. A bushing of any type gives a wider choice of materials with which to produce a long running bearing couple, but bushings also use up premium radial space which is a contraindication to their use in rock bits.
Another important area for improvement is in the nature of materials used in the construction of such bearings. Materials which are arc deposited are less than ideal choices being metallurgically non-homogeneous and of disordered microstructure.